You could understand it by reading the Intel® 64 and IA-32 Architectures Optimization Reference Manual
Short explanation for assists: when certain floating-point operations cannot be handled natively by the execution pipeline, and must be performed by microcode
At another page: SIMD FP assists deal[ing] with denormal inputs and underflow outputs.
To determine the costs of assists, you can use the performance events, see section 'B.4.5.2 Assists' (Version June 2016)
