Do you mean Friedel-Crafts Acylation? This is a form of elecrophilic substitution on a benzene ring using an acyl (or acid) chloride using a Lewis acid catalyst to form an aromatic ketone.
Ethanoyl (Acetyl) chloride in the presence of aluminium chloride as a Lewis acid catalyst reacts by the empty p orbital found in aluminium chloride AlCl_3 . This creates a positive charge on the acyl group and a negative charge on the chlorine atom. This acyl group accepts a pair of electrons from the six delocalised pi electrons and forms an intermediate which ejects a proton by two electons from a C-H bond being reforming the aromaticity of the benzene ring. The product in this case being phenylacetone.