ablative case

In my book, There is no clear definition of what the ablative case means, even though the term has been introduced for nouns, and we are now into prepositions. So far, we were given an example of the noun Nauta in the ablative, in singular and plural. Nauta (with a bar over the a) is singular, and Nautis (with bar over the i) is plural. Now for prepositions.
The Latin word "in". In my book, it says this word is a preposition, and can take on either the ablative case, or the accusative case. In the ablative case, the Latin word "in" translates to: in, or on. In the accusative case it means: "into".
It explains further; you can tell it is the ablative case because the following word will have a macron over it's last vowel, as in "insula" (imagine a bar over the a). So the Latin "in insula" (with a macron over the a) means "in the island", or "on the island".
On the other hand, take the Latin "in" in "in silvam". [ Silvam is without a macron, and obviously has direct object ending.] Here the Latin "in"takes on the accusative case, and translates to"into", and the phrase translates to "into the forest".
The question is what does a preposition in the ablative really mean?, (for that matter what does a noun in the ablative really mean?)
Staying with prepositions: A preposition in the accusative seems? pretty straight forward. It is tied to a direct object (and noun) and describes that direct object (and noun).
For example; Agricola in silva ambulat. (with macron over the a in silva), translates to "The farmer is walking in the forest." The "in" tells you where he is walking but seems (to me) more tied to (the direct object) forest than (the verb) walking.
The ablative, on the other hand, seems to more clarify a verb. For instance, Agricola in silvam ambulat. "The farmer is walking into the forest." The "into" tells you where he is walking (at the edge of the forest) and infers a destination (the forest). Therefore, into seems more tied to the verb walking.
Is there a clear definition for the ablative case of a preposition?
Is there a clear definition for the ablative case of a noun?
thank you