"So that I can confirm what does "+" and "-" value means in term of up or down regulation."

I would first say that up or down regulation is a mechanism, whereas your experiment is measuring an enrichment in one condition relative to another. No need to put forth a mechanism when you simple need to describe what you observe.

I get the sense you're more confused about numbers in log space than anything really having to do with edgeR. When you're comparing two things: A and B, the fold change is A/B. A and B could be data sets reflecting gene expression measured under different conditions. If gene1 is 2-fold higher in A, the A/B ratio for gene1 is 2. If gene47 is 2-fold higher in B than A, the gene47 ratio is 0.5. However, edgeR displays the log2 of the ratio. Therefore genes up or down by a given amount (i.e. two-fold) have the same distance from equality, but the sign (+/-) changes. Thus gene1 would have logFC: 1, and gene47 would have logFC: -1, reflecting 2-fold up or down respectively. In log2 space the numbers are close enough to how we think about fold change to develop an intuition. For instance: -2 is 4-fold down, -1 is 2-fold down, 0 is equal in A and B (a ratio of 1), 1 is 2-fold up, 2 is 4-fold up, etc. To convert the logFC value from edgeR (which are essentially ratios of the normalized count values per gene) into an up or down ratio, you simply take 2 to the power of the logFC number. e.g. for a gene with logFC of 0.5 or -0.5: the ratio is 2^0.5 = 1.4, or 2^-0.5 = 0.7, respectively.