The SAVE monitor command writes your current core image to a .DMP file on
disk so that it can be run later.  This is usually done just after loading
a program, to save a permanent version of it.  See the Monitor Command
Manual (online as MONCOM.BH[S,DOC]) for more information.

There is also a program named SAVE, which writes files to/reads files from
magtape in a simple format.  SAVE.REG[UP,DOC] documents the program, or
type /H to it.