How to update solr index?

When the user creates a document, I add the date to the solr index. Every time the data changes like edit or delete, do I have to reindex the whole data?

What does reindex mean in that case? When I do

$this->indexData(array(
        'id' => $pid,
        'title' => $data['titel']
));

for each document and do $solr->addDocuments, does it just overwrite already existing data?

I tried to reindex the whole index on add/delete/edit but after I delete a certain field its information still seems to be in the index.

Any ideas?