As a general term an "optimiser" simply means a system that produces the best result for a given set of inputs.
Many years ago, the only so-called television optimisers were ones that simply provided a set of programmes (or spots or breaks or advertising blocks) that achieved the largest number of GRPs for a fixed amount of money. This could be done simply by estimating the average GRP of each programme, dividing this by the cost of the programme, ranking the results and picking whichever programmes came highest. However, nowadays, it is commonly accepted that the true effectiveness of an advertising campaign is measured by its reach - not simply the total number of GRPs obtained. It is important to point out that other so-called TV optimisers of 10-15 years ago just used a model based on GRP levels per daypart to estimate reach - and this simply wasn't accurate enough and would often produce meaningless results. Around 1989 it was realised that any practical TV reach optimiser must use respondent level data.