Political action or belief which seeks to keep something in being. Etymologically, there is no significant difference between conserving something and preserving it in any of the languages which contain these two verbs. In Victorian England those who favoured what would now be called conservation tended to refer to the ‘preservation’ of the things they regarded as important (footpaths, ancient buildings, or species, for example) and what is now the Council for the Protection of Rural England was founded in 1926 with the word ‘preservation’ in its title instead of ‘protection’.
However, an important nuance has come to distinguish conservation from preservation: conservation accepts that you cannot literally keep things as they are, but only manage change to preserve what is valuable. Thus conserving a forest does not just mean preventing anyone from chopping down the trees, it means planting new trees and even new types of tree if that is what is needed in order to maintain a healthy forest.