Technical Indicators: A technical indicator (often referred to as TIs or 'technicals') is basically a series of data points which are derived form a historical data set of a given market or security. The data points are normally plotted in a chart which can provide a visual indication of where are market is headed. Technicals are often used in automated strategies. Examples include Moving Average, Relative Strength Index, On Balance Volume and many more.
Candles: Please see Bar Charts below.
Slippage: Slippage relates to the difference between the price that a trader expects to get executed at and the price that he actually gets. In forex, slippage will most often occur when a limit order or a stop loss ends up being executed at a price inferior to the one requested or set when opening the trade. Slippage is likely to occur in 'fast markets' when liquidity is poor and prices are more volatile than usual.
Latency: (Data) Latency relates to the time it will take a set of data to travel from A to B. In other words, the accuracy - or timeliness - of the data that you are using to trade may lagg realtime karket data and hence make your trading inaccurate. In forex trading the issue of latency is a difficult one to measure as there are typically several sources of latency, the primary ones being of application, network and geographic nature. Latency may or may not affect your trading results and will in large part depend on your trading style.
Rejections: Rejections in fx relates to the situations where a trade attempts to trade on a published price only to find that the broker refuses to execute the trade. There may be a number of reasons for this, the most common ones being insufficient funds in account, invalid user-id, missing permission to trade security in question, broker parameters not being matched.
Re-quotes: Requotes in FX refer to situation where an execution of order is requested at a certain price, but the broker returns the request with a different quote at a new (worse) price - a re-quote. Re-quotes mostly happen in fast markets with high volatility and poor liquidity.
Tick data: Tick data is the more accurate of the two and it refers to market data which shows the price and volume of every print. Additionally, tick data will often include every change to the best bid and ask price.
Bar Charts (OHCL): Bar charts, also referred to as OHLC charts (open-high-low-close chart), are a type of chart which help in illustrating movements in the price of a financial instrument over time. Each vertical line on the chart shows the price range (i.e. the highest and lowest prices) over a given time unit, e.g. 5 minutes, 30 minutes, 1 hour or one day.