What this is and what it's for
The 6ES7720-5AC42-0YC4 is a Siemens PG 720 P programming device — a ruggedized industrial computer used for commissioning, diagnostics, and programming of Siemens automation systems on the plant floor. The P150 processor and 2.1 GB hard drive put it in the mid-range of that generation of field programmers.
The description — PG 720 P, P150, 2.1 GB HD, 5spr. — identifies the processor class (P150) and storage capacity (2.1 GB hard drive). The "5spr." likely refers to a 5-slot expansion chassis or a 5-screw terminal variant, typical for connecting to fieldbus or I/O modules in a control cabinet.