What is the meaning of -j?

Attached to o-stem verbs (in active voice). Indicating 1st person singular; indicative, present.

Marks the dual form of nouns and adjectives. Attaches to the singular form if feminine or the plural form if masculine.

Sometimes added to suffix pronouns attached to dual nouns.

Converts nouns (including proper nouns and nisbas used nominally), prepositions, independent pronouns, and numerals into masculine adjectives: the masculine nisba adjective ending.

Forms i-stem masculine nouns from roots.

Forms prepositional adverbs from certain prepositions.

Forms the infinitive of anomalous verbs, weak verbs (except for fourth weak verbs with a geminated stem), and causative biliteral verbs.

Optionally marks the masculine imperfective active participle, intervening between the stem and the gender/number endings.

Optionally marks the masculine geminated perfective passive participle of strong biliteral verbs, intervening between the stem and the gender/number endings.

-s, -es (marks the plural form of nouns, adjectives, and some pronouns)

Forms the second-person singular subjunctive/imperative of verbs (indefinite conjugation). It is also part of all subjunctive/imperative suffixes in both indefinite and definite conjugations.

forms imperatives of vowel stems

Source: wiktionary.org