It means that the securities (as contracts or legal agreements) say, hold, or imply (that) . . . (when something happens, something else cannot happen).
The definition is as given here:
2 : to make a proviso or stipulation
the Constitution … provides for an elected two-chamber legislature ...
An additional definition with some examples is given here:
3 [INTRANSITIVE/TRANSITIVE] LEGAL to contain statements or plans that set conditions for dealing with a particular issue
provide for: The treaty provides for the immediate withdrawal of troops from the area.
provide that: Their contracts provide that they will be paid on the last day of the month.
ADDED: @StoneyB's supplementary comment is postpended here for convenience:
Provided that X and providing that X are participle clauses (some would actually categorize them as preposition phrases!) acting as clausal adjuncts: they are in effect conditions (="If and only if X is true") modifying the sense of the main clause. In the sentence you quote provide is the verb which heads the main clause ("our securities provide X").