Sūtra 1-28
तज्जपस्तदर्थभावनम्। (२८)
Tajjapastadarthabhāvanam.
(Tat-japaḥ tat-artha-bhāvanam) (28)
(May one do) Repetition of His name and contemplating on its meaning.

Sūtra words
tat-japaḥ and tat-artha-bhāvanam

Word Formation
tat-japaḥ — This is a ṣaṣṭhī (sixth case) tatpuruṣa compound, it is resolved as tasya japaḥ. tasya — is the sixth case singular, masculine gender of the pronoun tat. japaḥ — is the first case singular, masculine gender of japa, derived: from (verb) jap + (suffix) a
tat-artha-bhāvanam — is also a  ṣaṣṭhī (sixth case) tatpuruṣa compound. It is is resolved – tasya arthasya bhāvanam. tasya — sixth case singular, masculine gender of pronoun tat. athasya — sixth case singular, masculine gender of the noun artha, derived from (verb) ṛ + (suffix) a [the alternate formation arth+a is not of relevance here]. bhāvanam — first case singular, neuter gender of the noun bhāvana, derived from (verb) bhū + (causal suffix) i  + (suffix) ana

Meaning of words
Tat-japaḥ — repetition of that. The pronoun that refers to the previous aphorism praṇava (1-27), the name Patañjali gives to Īśvara (1-24).
tat-artha-bhāvanam — contemplation of its meaning.

The aphorism in itself is incomplete. The literal translation is based on the words – mental repetition and contemplation of its meaning. To complete the sense, one has to take this aphorism as an injunction meaning ‘may one do / one must do’. The translation listed here incorporates this convention.