The phrase "substantial upside" refers to the potential for significant or considerable positive outcomes or benefits. It means that there is a high possibility of achieving favorable results or rewards. Full definition
«There remains substantial upside in this portfolio,» he said. (nreionline.com)
Since you do not put anything down, and you can always get back at least what you paid, it seems like there is no downside and substantial upside if the flat appreciates. (eternalyield.com)
Based on FIG's $ 3.11 share price at that point, this offered substantial Upside Potential of 151 %. (wexboy.wordpress.com)