The first value refers to Cover conditions, the second one to Vinyl conditions.
For any item, the biggest faults (for example: scratches or surface marks) are described in detail, both for vinyl and record sleeve.

S or M = Factory Sealed or Mint.
As you should buy a new record in a shop.
M- = Near mint.
Record played a couple of times.
M-- = Well-preserved item.
Record played more times. Occasional very light noises on background.
Cover has light wear, usually more noticeable along the edges.

EX = Excellent.
Record played many times. Some light noises on background.
Cover has wear along the edges and on corners. Presence of ring wear.

VG = Very Good.
Record has light scratches and/or surface noises.
Cover is worn and shows defacement.

G = Very used record.
Scratches and surface noises.
Cover damaged.

F = Fair.
Noisy record.