I gather from this tweet that grabbing the mace lying on the table of the British House of Commons is an act of protest and has some kind of significance.
What significance does this gesture have?
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The December 10 incident was a protest that the UK parliament is no longer in control of crucial EU withdrawal negotations ("Brexit"). Several key votes have been suspended; the sitting government has been found in contempt of the chamber; and the prime minister, Theresa May, appears to have lost the confidence of her own MPs, creating an effectively headless government.
Therefore, the argument goes, parliament is no longer functioning and no longer has a right to wield the Queen's mace.
That mace represents the sovereign's authority - in other words, that parliament rules with the Queen's assent. Positioning the mace in the centre of parliament is a statement: the house of commons has that assent, has that power and remains in charge. So removing it is highly symbolic.
Taking the mace is rare but has generally represented one of three things:
1. Contempt for parliament.
Famously Oliver Cromwell, Lord Protector and dictator, removed the mace in 1653 during a conflict with parliament. He derided it as a "fool's bauble", had it taken away by armed troops and dissolved the house by force.
2. Breakdown of parliamentary authority.
Though Britain lacks a written constitution, convention is that the authority of MPs is sacrosanct. If key bills or statutes are passed without parliamentary assent, an MP may occasionally pick up the mace in a protest against abuse of (usually prime ministerial) power.
One example might be John McDonnell taking the mace in 2009: a highly controversial airport expansion had been pushed through without a parliamentary vote taking place. McDonnell felt this was a trespass against the house's authority and demonstrated that by moving the mace to an empty bench.
3. Violation of parliamentary norms.
Parliament has several traditions and informal rules that allow it to run smoothly. One example is 'pairing', where an MP for a motion may abstain if an MP against it must be absent due to illness, bereavement or an emergency. Occasionally a government or party may break these rules and face protests that they are 'fighting dirty'.
This is exactly what happened in 1976 when Michael Heseltine seized the mace: a breach in pairing protocol led to a key industrial bill winning by a single vote. Enraged, he took the mace in a protest over what he had felt were underhand tactics from his Labour opponents.
The mace is a symbol of the Queen's Authority. Its presence in the House of Commons signifies that the House has the Queen's authority to pass laws, etc.
It is not unknown for an MP to make some kind of protest by grabbing it, but they always seem to look a bit foolish as a result, and it never accomplishes anything except for a bit of light-relief in the news headlines.
I have also known MPs that have committed some kind of parliamentary misdemeanour to be made to "apologise to the mace".
Update: asked to clarify "what does the gesture mean?"
The gesture has no defined or agreed meaning. The only meaning is whatever was in the head of the person who grabbed the mace. In the most recent case he did explain afterwards why he had done it - something about the government having lost its privilege to rule - but I think it would be a different reason every time.