There is a wide variety of horseracing in the UK, and to make money at gambling on it you need to understand the different formats.
If we ignore specialism not usually gambled on in the UK (Arabian and harness races) the basic division is between Flat races and National Hunt (NH) races. Flat racing developed as an aristocratic amusement and as the title says these horse races are run on the flat without obstacles, and start from “stalls” the equivalent of a starting pistol in a human race.
The gentry developed NH races from farming and hunting, through “point-to-point” meetings. NH races involve the horses jumping obstacles. Whilst some courses offer both flat and NH meetings; while some horses race over the flat and the “jumps” and while some NH races are run over the “flat” they are different codes (akin to Union and League in Rugby.)
There are “handicap” and “non-handicap” events. 60% of races are handicap races. A handicap race is one where the “handicapper” allocates a least weight for the jockey to carry. Very broadly a pound in weight represents a length in distance in NH racing (with more variance in flat racing depending on distance.) Therefore if on form a NH horse should finish 5 lengths ahead of it rival, then in a handicap race it should carry 5 pounds more in weight then that rival.
Whether a jockey “can do the weight” is a consideration for the trainer choosing him. If he can’t then he is disadvantaged carrying “overweight”, but in most cases the underweight is made up by lead saddle weights. Jockeys and their saddles are “weighed in” at the end of a race to ensure that they carried the correct weight, and if not they can be disqualified.
The basic idea of a handicap is that all horses should finish together, therefore fewer favourites win handicap races than win non-handicap races. Rapidly improving horses can “get ahead of the handicapper” and on the other hand the handicapper is said to “have caught up with some horses” which can create profitable betting opportunities either way.
Some handicap races have a least weight which the horse has to carry even though the handicapper would allocate it a lower weight. Such horses are “out of the handicap” and face a disadvantage to their rivals. “Conditions” races are not handicaps but make allowances for a horses weakness (e.g. it is being ridden by an amateur jockey) or penalizes strengths (e.g. it has already won a race) or offer “age-for-weight” or allowances for which sex the horse is.
When studying form it is important to read the “conditions” attaching to such races.