What does mildly increased lymphoplasmacytic cells mean in my stomach biopsy results?

Common nonspecific. a completely normal stomach biopsy is supposed to show essentially no inflammation. However, it is extremely common to see very mild degrees of chronic inflammation (lymphocytes and plasma cells). Active gastritis due to helicobacter pylori would have acute inflammation, which was not reported in your biopsy. Your findings are non specific and common/minimal, does not need treatment.
Mild inflammation. The biopsy report may show other more important information like whether it is positive for Helicobacter pylori.