A Y-adaptor (also called Y-cable or splitter) is used to: send one output to two inputs (splitter), or combine two outputs into one input (combiner). Splitting is generally safe (one source feeding two destinations). Combining two outputs into one input is problematic — it connects two output stages together, potentially causing distortion or damage.
For combining signals, use a proper mixer or summing box — never a Y-cable. For splitting a line-level signal to feed two destinations, a Y-cable works but may reduce the level slightly due to impedance loading. Buffered splitters (active, powered) avoid this issue.