Complex sentence: The Pentium 4's trace cache stores micro-operations resulting from decoding x86 instructions, serving also as a micro-operation cache, so that the next time an instruction is needed, it does not have to be decoded into micro-ops again.
Split into simpler sentences: The Pentium 4's trace cache stores micro-operations resulting from decoding x86 instructions, providing also the functionality of a micro-operation cache. Having this, the next time an instruction is needed, it does not have to be decoded into micro-ops again.