I am new to the field of semantic knowledge representation and all I get information about the term semantics = meaning. Can somebody please explain to me what is semantics and how it is important in cognition.
Logical semantics is one of the three sections of logical semiotics (in addition to syntax and pragmatics) dealing with the relations between signs (in particular the expressions) and reality, to which they refer. In addition to basic semantic functions, we can include, e.g. semantic connotation, determination and denotation, where the basic terms of semantics are meaning and truth. For example, I recommend Benthem (1986) and maybe Wikipedia's definition at the beginning of your journey. For further reading I can also recommend Friedenberg & Silverman (2012).
How can the relation between signs, expressions, symbols, and reality, in which semantics works so hard, be referred to in cognitive science? One very simple (at the beginning) answer - representations. It is one of the fundamental processes in cognitive science; a representation is something that stands for something else. Single words can be good examples here, e.g., apple. When we say it, then in the mind of someone who knows English, the concept of a particular type of fruit will pop up. One will see it, taste it, and maybe touch it in his/her imagination.
There are other types of representations. For example there are propositions, which are statements about the world and which can be illustrated with sentences. The sentence, e.g John is eating a red apple is a proposition, that is itself made up of a few concepts. There are also rules, which are another type of representation, and they can specify relationships between prepositions. Let's say the rule, If it is raining, I will not go with you to ZOO makes the second proposition contingent on the first one. Also, there are analog representations, which help us make comparisons between two similar situations, objects, etc.