Short stay Schengen visa entitles the holder to visit the Schengen countries for tourism, visiting friends/family, business etc. up to 90 days within 180 days period from the date of first entry. The stay may be either continuous or several visits depending upon whether the visa is single or multiple entry.

Single-entry visa: Allows uninterrupted stay. Once the person departs the Schengen territory, he/she can't enter again without obtaining new visa.

Multiple-entry visa: Allows multiple stays in one or more Schengen countries, as long as the total length of the stay does not exceed 90 days in a given 180 days period from the date of first entry. The stay may either be continuous or spread over several visits within the validity periods of the visa, but no more than the number of entries authorized by the visa. Multiple entry visa may be valid for 1 year.
If the entries show "02", you are allowed 2 entries. If the entries show "MULT", you are allowed more than 2 entries.
Multiple entry visa is not issued at all the embassies/consulates.

If the validity of the visa is over while visiting the Schengen territory, the person can no longer stay in Schengen territory even if the person has not stayed for 90 days. Additionally, visa will expire when the total number of exits made by the visa holder equals the number of authorized entries, even if the visa holder has not used up the number of days authorized by the visa.