A trajectory is a sequence/history of state-action-reward information about an agents personal experience in its environment. A trajectory can be thought of as the history of an agent, all of their lived experiences, their story, their memory, their past, their knowledge.
A trajectory is basically the facts about what actions were taken in what states, what new states the agent entered into, the next actions they took in that state, and so on.
If an agent had infinite memory, they could remember every detail about their entire life trajectory: every state their body was ever in, every action they ever took, and how they perceived reward throughout that entire time.
Trajectories from different agents can be shared from agent to agent by communicating information. This can help an agent learn by comparing their own trajectories to trajectories from other agents.