I'm not a grammar expert, but I'll try to explain it.

When you're joining phrases with conjunctions (word like 'and' or 'or') you need to make sure that they have similar grammatical structures.

You should look out for faulty parallelism when you're using a construction like:

a and b
a, b, and c
a or b
a, b, or c
not only a but also b

Here's an example of a faulty parallelism:

He liked to play basketball and riding horses.

It sounds a lot less awkward if you change it to one of the following:

He liked playing basketball and riding horses.

He liked to play basketball and to ride horses.

Hope that helped!

p.s. It's spelled grammar, not grammer less