Wait actions allow you to delay further actions for a specific amount of time. This helps to keep actions in sync for presentation purposes, such as timing an Overlay Image with a specific cue in a song. The amount of time you specify is in seconds and can be a decimal.
The majority of actions do not wait for their work to complete (EX: A sound action only starts playing the sound, it does not wait for it to finish). The Wait action is the one major exception to this, amongst a few other minor ones (EX: The Web Request action must wait for the web request to complete because it needs the result to put into the Special Identifier output). Same-type serialization, where another Chat command can not run until the current Chat command has finished, is a separate mechanism and is not caused by the Wait action itself: it comes from Mix It Up’s default “Per Command Type” Command Locking System, which serializes any commands of the same type regardless of whether they contain a Wait action, unless you change the lock setting or individually Unlock the command.