Prajalpa is referring to talking unnecessarily or, talking about a mundane subject matter that didn't have any relation in devotional service and Krishna. All prajalpas that are unfavorable to Krsna’s service are adverse to devotional service The practitioner of Bhakti Yoga should carefully give up prajalpa. They are of many types. Useless talks, arguments, gossip, debates, fault-finding in others, speaking falsehoods, blaspheming devotees, and worldly talk are all called prajalpa.
In the Srimad-Bhagavatam (11.28.2) Lord Krsna instructed Uddhava as follows:
“Whoever indulges in praising or criticizing the qualities and behavior of others will quickly become deviated from his own best interest by his entanglement in illusory dualities.” [An Excerpt from Sri Bhaktyaloka by Srila Bhaktivinoda Thakura: