Shubh Muhurat means something like auspicious time.
In some cases it could be found out by kundali's, but not everyone believes in kundali matching.
Sometimes it's just a good period of a day when one has to do important tasks.
Since marriage is one of the most important things in life, people normally find out a shubh muhurat for that. Then there are others like naming ceremonies, housewarming, inaugurations etc. when people consider following the muhurat. It really depends on the individual though. In some cases people follow shubh muhurat to step out when going to see a girl for their son too 😉
So mostly shubh muhurat is some period (few hours maybe) of a day.. though there are some days like Dusshera which is considered shubh alltogether, so mostly people do not look for specific timeframe on such days as the whole days is considered good..
Hope this helps.. 😊