The Shalosh Regalim is the Hebrew collective term for the three Foot Festivals that are biblically mandated in Exodus
"Three times in the year all thy males shall appear before the Lord God." (Ex 23:17)

and more fully in Deuteronomy

"Three times in a year shall all thy males appear before the Lord thy God in the place which he shall choose; in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles: and they shall not appear before the Lord empty: every man shall give as he is able, according to the blessing of the Lord your God which he has given you." (Deu 16:16-17)

These three festivals are:

As the passages above implied, these festivals were times of pilgrimage, when all Jews were supposed to go to the Temple in Jerusalem with gifts, either of agricultural produce or money.

The regalim are often translated as 'foot festivals'. This is because the Hebrew word regel (hard 'g') means foot. However, the usual plural of regel is raglayim. There is one other time the word regalim is used:

And the Lord opened the mouth of the donkey, and she said to Balaam, What have I done to you, that you have hit me these three times? (Num 22:28)

The expression "three times", in Hebrew, is 'shalosh regalim'. This leads to one view that the name regalim has nothing to do with feet or festivals, but merely means "the three times".

Although there is currently no Temple in Jerusalem, Jews still observe all three of these festivals annually, practicing instead the personal and communal rituals, but skipping the sacrificing.