The Eucharist
It is Jesus himself, his Body and Blood, under the appearance of bread and wine. It is the center of the life of the Church.
The sign
Bread and wine, and the words the priest says: the same words Jesus said at the Last Supper.
Who gives it
Only a priest (or a bishop) can celebrate Mass and consecrate the bread and wine.
How many times
Many times. The Church encourages us to receive Communion every Sunday, and even every day. And she asks us to receive at least once a year.
What the Bible says
Luke 22:19-20
19And taking bread, he gave thanks, and brake; and gave to them, saying: This is my body, which is given for you. Do this for a commemoration of me. 20In like manner the chalice also, after he had supped, saying: This is the chalice, the new testament in my blood, which shall be shed for you.
Douay-Rheims Bible (Challoner revision), public domain.
Source: Catechism 1322-1324, 1374, 1389, and 1411-1412