Verbs are actions or states. They tell you whether something has already happened (past), whether something is happening at this moment (present) or whether something will happen (future). As well as telling you the tense (which is either past, present or future), they also tell you what is happening. I'll give some examples, putting the verbs in bold.
Peter played his harmonium.
You can see that at some point in the past, Peter had played his harmonium. If it was happening in the present it would be "Peter is playing his harmonium". Both "is" and "playing" are verbs. "Is" is a verb because it tells you that something is happening in the present. "Playing" is a verb because it tells you what is happening.
If Peter is going to play his harmonium at some point in the future the sentence would be "Peter will play his harmonium". "Will" is a verb because it tells you when an event occurs. "Play" is a verb because it tells you what will occur.
Past Tense: The cat slept in front of the fire.
Present Tense: The cat is sleeping in front of the fire.
Future Tense: The cat will sleep in front of the fire.
Past Tense: Peter lit a firework.
Present Tense: Peter is lighting a firework.
Future Tense: Peter will light a firework.
I hope that's been helpful to you.