Value bets are those bets that are "worthy" to be played. A bet is always a value bet when the odds too high in relation to the probability.
Example for value bets : In your opinion, to win the championship match of FC Bayern against Hamburg with a probability of 50%. Suppose, the bookie X provides for a win of Bayern at a rate of 2.10. With a 50% probability of a balanced ratio would have to be quoted with 2.00. Since the rate is obviously higher than the expected probability to value of 2.10, it is therefore a value bet. To predict the probability of a certain event, of course, exact studies and a degree of dexterity is required. These should be determined by both Statistics (various tables, Head to Head, etc.), as well as current influences (e.g. blocking players, shape, etc.).
Likewise, it is possible to calculate these probabilities using the Unibet oddset offered on the market. Here it is assumed that the average rate of the different bookmakers shows a true picture of the likelihood ratios. These average rates are in turn converted to percentages, so I get an average probability of the possible betting outputs. Now a bookmakers is well above this average, I see where the value and place my bet.
However, to calculate probabilities for sports betting, which also reflect the actual situation on the field requires much knowledge and experience. Value bets do not lead to secure profits, but these are certainly a betting strategy that is worth to be studied. Since value bets is the most basic of all sports betting strategies is a basic knowledge of advisable in all cases.