I am new to BioInformatics, I want to know what is gene expression level and how it is determined.. Anyone who can answer this silly question.
“Gene expression level” is an imprecise and partially ambiguous term that should be avoided.
Many (but not all) genes have the information to produce proteins†, but there is no need for this information to be used at any given time — it depends on the requirements of the organism or cell. For example the DNA of lung cells contains the information to produce the protein chains of haemoglobin, but it is never used in those cells. We can say that “the gene is not expressed”.
So the absence of expression is fairly easy. However, for a protein-coding gene the meaning and quantitation of expression is more difficult. That is because in expressing the information in the gene the DNA is first transcribed into mRNA by RNA polymerase, and then that information that encodes a protein can be translated into protein on the ribosome.
The word level refers to quantification, and is a sloppy substitute for quantity or perhaps rate (quantity produced in unit time) or perhaps relative quantity.
But the main difficulty is whether in quantifying gene expression you measure the quantity of mRNA synthesised under standard conditions or the quantity of protein produced.
If you are a bioinformatician encountering this term, my bet is that it is in relation to the currently (2018) popular RNA-Seq technique, which in fact provides relative measurements of the quantity of RNAs of different types produced under standard conditions. This is generally expressed in units called FPKM — Fragments Per Kilobase of transcript per Million mapped reads. The difference between this technique and the northern blotting mentioned by @Hawkeye, is that the latter is used to detect a single mRNA, whereas RNA-Seq applies to all the mRNAs that are present in a sample.
†Some genes encode RNAs that do not encode proteins (ribosomal RNAs, transfer RNAs, RNAs involved in splicing and in modification of rRNA, microRNAs etc.). The expression of the genes for these will end with the synthesis of the RNA.