Learn French: Mon bagage est très léger.

No, in French, bagage is a normal countable noun. Here it is singular (mon bagage), which refers to one bag or piece of luggage. If you had multiple bags, you would use the plural and say mes bagages.

Why is it léger and not légère?

Léger is the masculine singular form of the adjective for 'light' (in weight). Adjectives must match the gender of the noun they describe, which in this case is the masculine word bagage. If you were describing a feminine noun, like a suitcase (une valise), you would use the feminine form légère.

Why do we use mon for 'my' here instead of ma?

In French, possessive words like 'my' always match the gender of the object being owned, not the person who owns it. Because bagage is a masculine word, you must use the masculine form mon, even if you are a woman speaking.

Does the accent mark on très matter for pronunciation?

Yes! The accent pointing down to the right is called an accent grave. It tells you to pronounce the e with an open 'eh' sound, very similar to the vowel sound in the English word 'pet'. Also, remember that the s at the end of très is silent.