What does the word parallel mean when applied to listed items?

To be grammatically correct, items in a list should be parallel in construct; that is, they must be expressed in similar fashion.

The list below has items that are not parallel:

Participants will learn several skills in the workshop:

The list below has the same items in parallel construct:

Participants will learn several skills in the workshop:
or 
Participants will learn several skills in the workshop:

Always begin items in a list with the same type of word (noun, adjective, verb, etc.) to achieve parallelism.