Agni literally means "fire".  In Āyurveda one will frequently hear it referenced as the 'digestive fire'. It governs cellular intelligence and awareness, digestion, absorption and transformation of food into energy / consciousness, It also governs the nutrition of all dhatus and regulates body temperature.

When agni is balanced one experiences a good appetite, a feeling of lightness after meals, increased energy, clarity, and feels well nourished. When agni is low, one has poor appetite, feels heavy and lethargic, and has decreased energy.

Agni imbalance can bring about the following conditions:
Vishama agni: Gas, constipation, dry skin, cracking joints, sciatica, insomnia, receding gums, muscle spasms, cramps and other vata-type disorders.   
Tikshna agni: Dry throat, heartburn, hot flashes, acid indigestion, gastritis, colitis and other pitta disorders.  
Manda agni: Cold, cough, congestion, oversalivation, decreased appetite, allergies, edema, obesity, hypertension, diabetes, lethargy, excessive sleep, generalized weakness in body, and kapha disorders.                                                                                                       

Agni imbalance also expresses emotionally and mentally:
Vishama agni: Nervousness, anxiety, fear, loneliness and isolation.
Tikshna agni: Anger, hatred, jealousy, envy, competitiveness, comparison & judgment.
Manda agni: Depression, attachment, grief, greed, possessiveness.

As you can see, the signs of agni imbalance are quite reflective of the associated dosha.

Here are general guidelines to treat agni imbalance:
Vishama agni: Follow a Vāta pacifying lifestyle and diet, especially regularity of routine meals and dinacharyā.
Tikshna agni: Follow a Pitta pacifying lifestyle and diet; avoidance of activities that overheat the body and mind (anger, etc.).
Manda agni: Follow a Kapha pacifying lifestyle and diet; exercise to decrease kapha. Eat more spicy foods.

What is Āma?

Low agni leads to the formation of āma, undigested foodstuff that stays in the system.
It is a toxic substance that is the root cause of all disease caused by low agni.

Habits that can cause low agni and āma:

• Overeating, snacking on anything but fruit between meals.

• Eating without real hunger or emotional eating.

• Drinking fruit juice or excess water during a meal.

• Drinking chilled or iced liquids at any time.

• Eating when constipated or emotionally disturbed.

• Eating before 7:00am or after early evening.

• Eating too much heavily processed food.

• Eating incompatible food combinations.

• Eating leftover food.

Some telltale signs of āma in the body:  

• Blocked and clogged channels (srotamsi)

• Lack of strength, low energy, fatigue, chronic fatigue.

• Obesity, lethargy, indigestion

• Restlessness, hyperactivity (due to ama in the nervous system)

• Congestion, increased or excessive salivation

• Accumulation of waste, constipation and bloating

• Decreased taste and appetite

• Dark or discolored urine, burning urination

• Generalized body ache

• Coated tongue, bad breath

• Foul smelling wastes (sweat, feces, urine)

• Sexual debility

Some of the doshic expressions of āma in the body are:

VĀTA

The tongue has a brownish color, vishima type digestion, constipation, low agni, stupor, increased peristalsis, abdominal pain, symptoms are worse at dawn and dusk (vāta time of daily cycle); restlessness or hyperactivity.  

PITTA

The tongue has a yellowish color; foul smell, acid indigestion and heartburn.

KAPHA

The tongue has a whitish color; congestion, increased salivation, sticky and cloudy mucus, and a suppressed appetite.

Yena dosha dūshyam sammūcchanāvasthā janitena bhāvi jvarādi mātram,
pratīyate na tu vātādi jatitatvādi visheshah.
Madhāva Nidānam

The remnant of the food, not properly digested because of the poor agni, which becomes the cause of all diseases is known as āma.