Upslope is a term to describe the beginning a weld cycle when the current is gradually increased from some lower value to a higher value over a period of time usually 3-10 cycles in standard AC welding. At the end of the upslope period then normally the full desired current is applied for the desired length of time.
Upslope is shown in the first section of the red current line - as it increases. This period is often used on galvanized materials to allow the zinc to melt and be pushed out of the joint. In other cases it is a chance for the components to anneal or relax and get all material into full contact before full power is applied.
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Reference: RWMA - Resistance Welding Manual 4th Edition