The first sentence वह लड़की चल रही है - 'That girl is walking' is in the present continuous tense and means that the girl is currently walking at the moment of speaking. Eg:वह अब धीरे से चल रही है (She is walking slowly now).
In addition, just like in English, the present continuous may also be used in place of the future tense for talking about fixed events in the near future.
The second sentence should be 'वह लड़की चलती है' because the verb must be conjugated as feminine (चलता would be masculine). This sentence is analogous to 'That girl walks' and is in the simple present tense. It means that she habitually or regularly walks but is not necessarily walking right now. Eg: वह रोज़ तीन किलोमीटर चलती है (She walks 3 kilometers every day).