This is the endorsement/restriction portion of the e-ticket, where a computer or an agent can insert text about the restrictions/changeability/miscellaneous comments about the ticket.

Typical text in the endorsement section looks like "NONREF" (ticket is nonrefundable or "UA ONLY" (ticket is only valid for travel on UA-operated flights) or "NONREF/NOVAL AFTER DPT" (ticket is nonrefundable, has no value after departure). The text is shorthand that can help people looking at the ticket quickly know some things about the trip's changeability. It doesn't actually set the rules for your trip – those are set in the fare rules — but people may only look at the endorsements section if they want to quickly know a summary of the full fare rules.

This text is sometimes inserted by a computer (if you buy a ticket on united.com, the text will be set to something based on your fare, which is why you often see very similar messages there). This text can also be written by a human. I have no guess whether what you see here was written by a person or by UA computers.

I'd interpret this text as meaning:
CHECK FARE NOTE: the refundability/changeability of this ticket is kind of complicated, so please read the full fare rules to understand how it may be changed/cancelled/refunded and what fees apply. (A simpler ticket might just say NONREF here if there are a very standard set of fees and rules that apply that are implied by that string)
OSL POS: I'm gonna guess this ticket was issued in Norway — is it a round trip originating in OSL? Did you use a Norwegian travel agent? (Booking something with Wideroe?)
END M: I have no idea. If a human typed this string manually, this might be their way of ending this part of the string, like a -30-.

By way of comparison, a sample auto-generated AA eTicket endorsement/restrictions area looks like "VALID AA/OAL ONLY/VOID IF SLD/PRCH/BRTRD/CHG FEE".