The subject is the compound gotamasavaka (disciples of Gotama, nominative plural). The verb is pabujjhanti (awaken, 3rd person, plural, active indicative, present tense). It has two attributes, the adverbs suppabuddhaj (well "awakenedly") and sada (always). There is a clause dependent on the subject: yesaj diva ca ratto ca niccaj kayagata sati (whose mindfulness is day and night constantly directed to the body). The subject is the noun sati (mindfulness, nominative singular). It has an attribute, the compound kayagata (directed to the body, nominative singular). The verb is omitted, implying the verb "to be". It has three attributes, the adverbs niccaj (constantly), diva (by day) and ratto (by night). The last two of these are connected by two conjunctions ca (and). The relative adverb yesaj (whose, genitive plural) connects the clause to the subject of the main sentence.
The story for this verse is identical with the one for the previous verses (DhP 296, DhP 297 and DhP 298) and the following ones (DhP 300 and DhP 301). This is perhaps the most important and most usual form of meditation - mindfulness of the body. We are trying to be mindful of all bodily movements, of breath, of every posture and intentions to move. There is one whole text in the Pali Buddhist Canon (Satipatthana-sutta) dealing with this meditation.