Locked posts can't be:

• Edited,
• Voted on,
• Commented on,
• Answered (if the post is a question),
• Flagged with most flags. ("In need of moderator intervention" flags are sometimes still allowed.)

In general, locks are used whenever modifications to a post are causing serious problems on the site. See the reference(s) below.

Reference:

• What is a locked post? in the Help Center
• What is a historical lock, and what is it used for? on Meta Stack Exchange