Topic is subject or main idea.

When you read,

it's important to be able to identify the topic

you're reading about.

Look at this text.

All the sentences describe lava.

That means lava is the topic or main idea.

All the sentences talk about penguins,

so the topic word is penguins.

All the sentences talk about Big John Henry,

so the topic is Big John Henry.

All the sentences talk about twins,

so the topic word is twins.