UL Grant is a specific physicall controntrol channel information from Network (eNodeB) telling a UE "Now you can transmit data" (More accurately saying "You can transmit the data 4 ms after you got this grant").
UL Grant is another name of DCI format 0. (Many people get confused by the name of "DCI format 0". They think DCI format 0 would be some information about downlink data transmission, but keep in mind that DCI format 0 is a control information about uplink data transmission).
UL Grant (DCI format 0) carries the following information and the most important information is 'Resource Allocation' and MCS. UE should transmit the data using RBs and MCS specified in this DCI 0.