SRS means Sounding Reference Signal, it is reference signal for eNodeB (sent by the UE) to figure out the channel quality of uplink path for each subsections of frequency region.
Why SRS is required?
ENodeB often allocates only a partial section of full system bandwidth for a specific UE and at a specific time. So it would be good to know which section across the overall bandwidth has better channel quality comparing to the other region. In this case, Network can allocate the specific frequency region which is the best for each of the UEs. (If we always have to use full bandwidth, we may not need this kind of reference signal since there is no choice even when there is a better or worse spots within the bandwidth).
How often UE transmit SRS ?
It depends on the configuration set by the signaling message (SIB2, RRC Connection Setup, RRC Connection Reconfiguration etc), but UE can transmit it every two subframes at the most and every 32 frame (320 subframe) at the least (10 bit signaling parameter srs-ConfigIndex tells UE of the periodicity of SRS transmission and the period can be 2,5,10,20,40,80,160,320 ms). Actually there is an option in which UE does not transmit SRS at all.