Explanation of the terms "downstream signaling" and "upstream signaling"
It simply means after and before with respect to the flow of information in a given pathway. For example, consider this schematic representation of a pathway:

TF1 ==activates==&gt; gene1 ==produces==&gt; Kinase1 ==phosphorylates==&gt; ProtA 


In this schema, Kinase1 is downstream of TF1 and upstream of ProtA. Or, to take a classic example (source):

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MEK1/2 is downstream of Raf and upstream of ERK1/2.