Bottom fishing is simply buying the cheapest investments (in terms of valuation ratios) available. Bottom fishing is value investing concentrated on the very cheapest companies. The term can be derogatory as it can imply a lack of attention to the quality of the investments selected.
Bottom fishing strategies can generate very good returns because the low valuations mean there is a lot of room for re-rating. There are usually good reasons why shares have low valuations and picking very cheap shares can be dangerous as many of them are cheap to reflect high risk.
Bottom fishing is very likely to lead investors into value traps.