A1 is the designation of an amplifier, in this case one with differential inputs (plus and minus, or non-inverting and inverting) - probably an operational amplifier. 'Massa' is the signal ground level. 'Ligt aan massa' could mean either 'is tied to ground' (if it is permanantly connected to ground) or 'is at ground level' (which simply describes the signal level at the non-inverting input). The latter is the safer option in the absence of additional context.