To be uncertain is to be unsure of something. Also, uncertain things are undecided, unknown, or doubtful in some way.

If you're certain of something, you're sure of it. When you're uncertain, you don't know. If you don't know the answer to a math question, you're uncertain. If you can't remember where you parked your car, you're uncertain about its location. Also, anything unknown or up in the air in uncertain. Trying to be a famous actor is an uncertain path. Gambling money on horses is very uncertain. And, it's certainly hard to plan for an uncertain future.

Definitions of uncertain
  1. adjective
    lacking or indicating lack of confidence or assurance
    uncertain of his convictions”
    “moving with uncertain (or unsure) steps”
    “an uncertain smile”
    “touched the ornaments with uncertain fingers”
    synonyms: incertain, unsure
    uncertain or unable to decide about what course to follow
    doubtful, dubious
    fraught with uncertainty or doubt
    acting with uncertainty or hesitance or lack of confidence
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    certain, sure
    having or feeling no doubt or uncertainty; confident and assured
    confident, convinced, positive
    persuaded of; very sure
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  2. adjective
    not established or confirmed
    synonyms: unsealed
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    certain, sealed
    established irrevocably
  3. adjective
    ambiguous (especially in the negative)
    “she spoke in no uncertain terms”
    having more than one possible meaning
  4. adjective
    subject to change
    “the weather is uncertain
    synonyms: changeable, unsettled
    liable to or capable of change
  5. adjective
    not consistent or dependable
    “an uncertain recollection of events”
    undependable, unreliable
    not worthy of reliance or trust
  6. adjective
    not established beyond doubt; still undecided or unknown
    “an uncertain future”
    “a manuscript of uncertain origin”
    “plans are still uncertain
    “changes of great if uncertain consequences”
    “without further evidence his story must remain uncertain
    not decided or not known
    up in the air
    very uncertain
    indeterminate, undetermined
    not precisely determined or established; not fixed or known in advance
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    established beyond doubt or question; definitely known
    known for certain
    indisputable, sure
    impossible to doubt or dispute
    sure as shooting
    absolutely certain
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  7. adjective
    not certain to occur; not inevitable
    “everything is uncertain about the army”
    “the issue is uncertain
    chancy, flukey, fluky, iffy
    subject to accident or chance or change
    uncertain because of uncontrollable circumstances
    up in the air
    not yet determined
    not capable of being foretold
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    certain, sure
    certain to occur; destined or inevitable
    bound, destined
    (usually followed by `to') governed by fate
    doomed, fated
    (usually followed by `to') determined by tragic fate
    foreordained, predestinate, predestined
    established or prearranged unalterably
    in for
    certain to get or have
    capable of being foretold
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