。
Eggs go bad easily in summer.
Literal
Egg [topic-は] summer [in-に-は] spoil-easy.
~やすい attached to a masu-stem forms an adjective 'easy to X / prone to X.' 腐る ('to rot / spoil') → 腐りやすい ('prone to spoiling'). The 夏には stacks に (location in time) + は (contrastive topic), a common pattern to mean 'in summer (as opposed to other seasons).'